Methods of preparing a work piece having a copper or copper alloy surface, more specifically a copper clad printed circuit board material, for subsequent coating the surface thereof with a polymeric deposit, more specifically with a photo imageable resist, are well-known in the art. At various stages in the process of manufacturing printed circuit boards, resist deposits are coated to the copper surface of the printed circuit board material and must excellently adhere to the copper base. For example, in creating copper structures, i.e., lines as well as bonding and soldering pads, a photo imageable resist is used to define these structures. Furthermore, after these copper structures have been created, a solder mask is applied to the structures in those regions which shall not be soldered. In both cases, the resist is applied to the copper surface and must well adhere thereto both during the imaging process (exposing and developing) and during any subsequent process steps, like copper plating (in the course of copper structure generation) and soldering.
For this reason, pre-treatment of the copper surfaces is at all events to be performed in order to prepare the copper surface for a good resist reception and hence excellent adherence thereon. Etching solutions are used for this purpose, such as for example solutions containing an oxidant for copper, like hydrogen peroxide, sodium peroxodisulfate or sodium caroate. Etching has generally been considered indispensable because etching is used to roughen the copper surface. This is because roughening has been considered requisite to achieve good adherence of the resist to the copper surface.
An example for such etching solutions is disclosed in WO 02/04706 A1. The etching solution described in this document is acidic and contains hydrogen peroxide, at least one five-membered nitrogen containing heterocyclic compound and additionally at least one microstructure modifying agent which is selected from the group comprising organic thiols, organic sulfides, organic disulfides and thioamides. The five-membered nitrogen containing heterocyclic compounds are tetrazoles and the derivatives thereof, such as 5-aminotetrazole and 5-phenyltetrazole. The microstructure modifying agents are for example selected from the group comprising L- and DL-cysteine, L-, DL- and D-cystine, 2-aminoethanethiol, mercaptoacetic acid, 3-mercaptopropionic acid, 2-mercaptoethanesulfonic acid, 3-mercaptopropanesulfonic acid, bis-(2-aminoethyl)disulfide, dithioacetic acid, 3,3′-dithiodipropionic acid, 4,4′-dithiodibutyric acid, 3,3′-dithio-bis-(propanesulfonic acid), thiodiacetic acid, 3,3′-thiodipropionic acid, thiourea, thiobenzamide and the salts thereof. Pre-treatment of the copper surfaces is performed to achieve good adherence of plating resists, etch resists, solder masks and other dielectric films thereon. Though little etching off of copper has been an object in this document in order to achieve low copper thickness variation due to the etching off, 10% etching off of the copper relating to the overall thickness of the copper layer is still required to achieve good adherence. Furthermore, a variety of other etching solutions are mentioned in this document, which also contain hydrogen peroxide or another oxidant for copper.
Further, EP 0 890 660 A1 discloses a microetching agent for copper or copper alloys. This agent also contains hydrogen peroxide, further sulfuric acid and, in addition, at least one compound selected from the group consisting of tetrazoles and tetrazole derivatives. More specifically the tetrazole derivatives may be 1-methyltetrazole, 2-methyltetrazole, 5-aminotetrazole, 5-amino-1-methyltetrazole, 1-phenyltetrazole and 5-phenyltetrazole. This solution is used to roughen the copper surface of a printed circuit board by microetching and imparting deep, biting ruggedness in the copper surface of a depth of 1 to 5 μm.
U.S. Pat. No. 3,645,772 discloses a process for improving bonding of a photoresist to copper. This process comprises scrubbing plastic boards having a copper foil with pumice, rinsing and drying same and then treating the roughened copper foil with a nitrogen-containing organic compound, which contains at least one other hetero-atom and a five- or six-membered ring. Preferred other hetero-atoms are reported to be nitrogen, sulfur, oxygen and selenium. Such heterocyclic compounds having nitrogen and one other hetero-atom may be 2-amino-6-methyl-benzothiazole, 2-mercaptobenzimidazole, thiobenzanilide, benzotriazole, 2-mercaptobenzothiazole, 2-mercaptothiazoline, 3-amino-1,2,4-triazole, 5-aminotetrazole monohydrate, tolutriazole, 1,2-naphthotriazole, 1-phenyl-3-mercaptotetrazole, 2-guinidinobenzimidazole and 1-chlorobenzotriazole.
The aforementioned etching solutions, however, are not suitable to be used in recent most sophisticated processes in which finest lines and other structures on the printed circuit boards are generated, like 5 μm lines and 5 μm spaces. In order to produce such ultra-fine circuitry, very thin copper is plated prior to forming these structures by etching. As copper in these processes is deposited by electroless plating, thickness thereof is about 1 μm only, for example. Meanwhile, using the above conventional microetchants, copper will be removed to a depth of at least 1 or 2 μm. For this reason, there will be the risk to totally remove the copper layer in at least part of the region on the surface due to the microetching step. This of course, will not be acceptable. For this reason, etching is considered detrimental to the consistency of the copper base.
In addition to including nitrogen containing organic heterocycles such as imidazoles, triazoles or thiazoles into the resist material, to attain good adherence of a resist to copper surfaces, K. H. Dietz in: Dry Film Photoresist Processing Technology, Electrochemical Publications Ltd., 2001, reports on using antitarnishing agents. Such agents may be strong or mild, the strong ones being benzotriazole and the derivatives thereof and the mild ones being hydroxycarboxylic acids such as citric acid. It has yet been ascertained that benzotriazole is not effective as non-etching adhesion promoter because it can only react with the metal surface and not with the photo imageable resist.
Further, EP 1 209 253 A2 discloses a process for treating adhesion promoted copper surfaces with epoxy resins in order to promote adhesion between a patterned circuitry innerlayer and a partially cured dielectric substrate material layer to form a multilayer. The process comprises treating the copper surfaces with an adhesion promotion composition first and an epoxy resin composition thereafter. The adhesion promotion composition may contain an oxidizer, an acid and a corrosion inhibitor. The corrosion inhibitor may be selected from the group comprising pyrroles, azoles, oxazoles, thiazoles, pyrazoles, triazoles, benzotriazoles, tetrazoles, tolytriazol, hydroxy-substituted azole compounds, imidazoles and benzimidazoles. Additionally the composition may contain a quaternary ammonium compound.
US 2006/0210819 A1 discloses polyimide composite coverlayers derived from two-layer polyamic acid-based composites having a cover layer and adjacent thereto an adhesive layer, wherein the adhesive layer comprises a polyimide base polymer and initially a polyamic acid precursor to the adhesive layer polyimide base polymer. The adhesive layer may be formed from a composition which may contain, inter alia, an adhesion promotor which may contain a heterocyclic or mercaptan compound. Useful adhesion promotors include 2-amino-5-mercaptothiophene, 5-amino-1,3,4-thiodiazole-2-thiol, benzotriazole, 5-chloro-benzotriazole, 1-chlorobenzotriazole, 1-carboxy-benzotriazole, 1-hydroxy-benzotriazole, 2-mercaptobenzoxazole, 1H-1,2,4-triazole-3-thiol and mercaptobenzimidazole.
WO 2005/033364 A1 discloses a method for pickling metallic surfaces by using an acidic solution containing an alkoxylated alkynol and optionally nitrogen containing polymers which comprise quaternary ammonium compounds having for example the following chemical formula:{—N+(CH3)2Cl−—(CH2)3—NH—CO—NH—(CH2)3—N+(CH3)2Cl−—(CH2)2—O—(CH2)2—}x 